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1.3 Treatment (Topic c)

Only 2% of cities in sub-Saharan Africa have sewage treatment, and only 30% of these are operating satisfactorily (UNESCO IHP 1996). The significance of wastewater treatment and disposal: that of protecting the public health and the environment seems not yet fully appreciated in the region. Much public education and creation of awareness is needed both for the decision makers and the public at large.

1.3.1 Large-scale technologies

1. Djibouti relies on the African Development Bank (ADB) to finance the sanitation master plan completed in 1988. It consists of construction of 8.3 km of main collector, a lift station, expansion of the treatment plant, and rehabilitation of the existing network. The treatment plant at Douda is designed for connecting 25,000 inhabitants, with possibility of extension to as many as 31,000 inhabitants. The treatment process is essentially an activated sludge process. The goal of the treatment is reduction of BOD₅ to 30 mg/l, destruction of pathogens to 99.9%, and elimination of solid particles larger than 100microns (World Bank, 1991).

2. South West Africa/Namibia is severely affected by water scarcity. Windhoek, the capital had a population of about 100,000 during a study which ended in 1984. Reclaimed wastewater was introduced in Windhoek in 1969 to overcome the effects of a serious water shortage as a result of prolonged drought. The water reclamation plant was established to augment the city’s inadequate domestic water supply. The scheme was based on a clearly defined policy of total wastewater reclamation strategy and entailed three integrated lines of defence.

• Sewage catchment quality control is based on diversion of industrial discharges containing potential harmful chemical compounds from domestic sewage collection systems;

• Efficient reclamation technology backed by vigilant control: efficient removal of pathogenic micro-organisms, toxic metals and organic compounds, which may have detrimental effects.

• Vigilant surveillance of the final water produced: determination of its microbial and chemical quality and use of early warning systems, based on biological sensors such as fish.

The Reclamation Plant was commissioned in 1969. It was designed:

• to treat 4,500m³/day

• to introduce an activated sludge plant at the Gammams Sewage Works

• for effluent low in ammonia-nitrogen.

The main stages of wastewater treatment are represented by settled sewage (99.7% virus positive, n=319), humus tank effluent or secondary sedimentation (93.7% positive, n=79) and maturation pond effluent (39.1%, n=156). This is followed by action of reclamation plant which performs breakpoint chlorination to produce the final water (0% virus, n=76). Then raw water intake from the dam is then received into the reclamation plant.

3. Harare, Zimbabwe: Harare has been described as unusual among cities of developing countries in the degree to which its sewage is treated (Porter et al., 1997). All Harare’s five sewage treatment plants undertake primary treatment of the sewage, and at least half of the city’s wastewater receives more advanced treatment. The product of the costly treatment is water free of pathogenic bacteria, but too high in nitrates and phosphates. It is now been diverted to municipal farms for irrigation of pastures and crops. The city is known to have spent more on sewage treatment than on the sewerage reticulation system.

4. The wastewater treatment philosophy adopted in Botswana has been the least cost but technically affordable state-of-the-art waste stabilization ponds. Diminishing land now predisposes capital investments more into the conventional sewage treatment systems.

Sewer flow in Gaborone is 18,000 to 75,000 m³ per day. It is deposited in stabilization ponds, covering 52 ha where treatment occurs through natural processes, with no machinery or energy input except for solar energy. As a result

• there is a reasonably high standard of treatment

• effluent is not fit for drinking, but some of it is used for irrigation, while some is further treated and discharged back into the Notwane River.

A switch is being made to sophisticated sewage treatment, in view of shortage of land for the lower-cost option currently in use.

5. The central sewerage system has been described earlier for Abuja, the new Federal capital of Nigeria. The temporary plant now in use has capacity to serve only 50,000 people, that is, 25% of its present population. It is therefore not able to provide adequate treatment of the wastewater. The Nigerian law on the quality of effluent from treatment plants requires BOD₅ of 20mg/l and suspended solids of 30mg/l, while coliform bacteria is reduced by 99%.

Extended aeration (activated sludge) is the method of treatment of wastewater from the Wupa drainage basin of Abuja city which serves 200,000 inhabitants. After the partial auto-oxidation of the wastewater, it is supposed to be oxygenated with diffused air mechanism and retained in the aeration tank for approximately 24 hours. But as noted earlier, because the capacity of the temporary treatment plant (contracts for the first of the five proposed treatment plants was awarded in 1989) is only 25% of the required size, aeration of the waste water could only be done for only a very short time. The result is a partially treated wastewater with BOD₅ of 30-40mg/l.

In many African cities some industries often locate outside the main industrial areas. In such cases, pretreatment requirements may be imposed; effluents produced by such industries may have to be pretreated in order to ensure compatibility with domestic wastewater before being treated together. In Abuja, such outliers are expected to comply with such requirements.

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